You may have to tell Alexa in the garage to connect to the extra wifi. I have a 2.4ghz network that is for my smart bulbs, and a 5 ghz for all other Alexa activities. I have to make sure both my Echos are connected to the same network to stream music through my combined speakers group.
However, do you really want them on separate wifi networks? Is your problem really that you need a combined speaker group, or that the garage needs it's own router?